Optical amplifier, optical transmission equipment, optical transmission system, and method thereof

ABSTRACT

An optical transmission equipment for use in an optical transmission system, having an optical amplifier ( 10 A), comprising: a first optical doped fiber ( 1 A); a second optical doped fiber ( 1 B); a third optical doped fiber ( 1 C); an optical isolator ( 6 ) of bringing loss in the optical signal, being provided between the first optical doped fiber and the second optical doped fiber; a dispersion compensator ( 7 ) being provided between the second optical doped fiber and the third optical doped fiber; and a pumping light source ( 2 ) being optically connected to so that the optical doped fibers ( 1 A,  1 B,  1 C) are excited in common.

[0001] This application is a continuation-in-part application ofapplication Ser. No. 09/129,844, filed Aug. 6, 1997 entitled “OpticalTransmission Device and Optical Transmission System Employing the Same”.And, U.S. Pat. No. 5,831,754 entitled “Optical Amplifier”, applicationSer. No. 08/432,074, filed May 1, 1995, patented on Nov. 3, 1998, andalso the above application entitled “Optical Transmission Device andOptical Transmission System Employing the Same”, application Ser. No.09/129,844, filed Aug. 6, 1997 are hereby incorporated herein byreference.

BACKGROUND OF THE INVENTION

[0002] 1 Field of the Invention

[0003] The present invention relates to an optical amplifier, an opticaltransmission equipment, an optical transmission system, and a methodthereof and in particular, to an optical amplifier, an opticaltransmission equipment, an optical transmission system, and a methodthereof, with which can be obtained a low noise figure (NIF) as well asdispersion compensation for an optical signal therein.

[0004] 2 Description of Prior Art

[0005] In general, it is already known that an input loss of an opticalsignal at a stage in front of a doped fiber causes Deterioration in aSIN ratio thereof, in particular in an optical amplifier using such thedoped fiber therein. However, as is described in “Optical Amplifier andApplications thereof” (published by Ohm Co. Ltd., May, 1992) 5-3[1], itis indispensable to insert an optical isolator at the front stage of thedoped fiber in the conventional optical transmission equipment, for thepurpose of suppressing the reflection of stimulated or induced emission(i.e., ASE: amplified spontaneous emission) light. In the opticaltransmission equipment with such the construction thereof, however,optical parts necessitated to be inserted at the front stage of thedoped fiber are not only the optical isolator. Namely, in general, theyalso includes optical parts, including a wavelength divider forwavelength of a supervising light, a coupler for monitoring strength ofa transmission signal, a multiplexer for multiplexing an pumping orstimulation light, etc., and they have respective losses therein. Forinstance, for obtaining a gain from 25 dB to 35 dB, it is necessary tocombine a semiconductor laser of about 100 mW for excitation and a dopedfiber of length from 20 m to 30 m, and in that case, noise figure (i.e.,noise index; being abbreviated as NF hereinafter) of the doped fibercannot be neglected.

[0006] In the optical transmission equipment with such the construction,the optical signal which is once damaged or receive losses therein on atransmission path or line is amplified by using the doped fiber havinghigh NF, after being further damaged or lost thereon, therefore it isdifficult to keep the NF less than 6 dB, which can be defined by a ratiobetween the S/N ratio of an input side and that of an output side.

[0007] Further, in a case where an optical signal of high velocity isapplied onto an optical path of ordinal transmission fiber (NDSF:Non-Dispersion Shifted Fiber), there is a necessity of inserting adevice for compensating the dispersion. As the result of this, there iscaused another necessity for compensating the loss due to that devicefor the dispersion compensating.

[0008] An example of the structure of such the optical amplifier ofconventional art is disclosed in a publication, “Trial of 2×2Bi-directional Relay Optical Fiber Amplifier (BDLA)” (1997 SocietyConference of Electronic Information Communication Society, B-10-184),with which the NF can be suppressed at 7.5 dB. Furthermore, a structuralexample is disclosed for example in U.S. Pat. No. 5,831,754 (JapanesePatent Laying-Open No. Hei 7-301831 (1995)), for compensating the lossdue to the disperse compensating device.

[0009] In case of transmitting the optical signal by multiple relays orrepeaters with use of k optical amplifiers, the deterioration amount inthe S/N ratio rises up in proportional to the number of the stages k.Therefore, in an actual optical transmission system where there is anupper limit in total amount of the S/N ratio deterioration, the numberof the relay or repeater stages must be lessened following the increasein the S/N deterioration amount of the optical amplifier. As a result ofthis, the distance of the optical transmission must be shortened.

[0010] For instance, under regulation of total amount of the S/Ndeterioration ratio to be equal to or less than 12 dB, if an opticalamplifier of S/N ratio deterioration at 4 dB and an optical amplifier ofS/N ratio deterioration at 6 dB are positioned at a distance 80 km,respectively, then the total S/N ratio deterioration amount comes to be12 dB for the transmission path relayed or amplified with three (3)stages of the optical amplifiers of 4 dB, while the same total S/N ratiodeterioration amount of 12 dB is obtained by relaying with two (2)stages of the optical amplifiers of 6 dB. Namely, the optical signal canbe transmitted at the distance 240 km with the optical amplifiers of 4dB in S/N ratio deterioration since it can be relayed at three (3)stages therewith, while it can be transmitted only at the distance 160km with the optical amplifiers of 6 dB since it can be relayed at onlytwo (2) stages therewith.

[0011] Though the S/N deterioration amount is not one beingcorresponding to the NF one by one, however, it becomes large when theoptical amplifier inferior in the NF is applied to, therefore there is aproblem that a distance for regenerative relaying or repeating comes tobe short, in which the optical signal is returned once into an electricsignal to be relayed or repeated.

[0012] Furthermore, with the optical amplifier in which the doped fiberis divided into a plurality of stages, a plurality of exciting orpumping light sources are necessary, therefore bringing about a rise-upof cost of the optical amplifier, as well as the large-sizing andincrease in electric power consumption thereof.

SUMMARY OF THE INVENTION

[0013] Accordingly, a first object of the present invention is, fordissolving such the problems as mentioned in the above, to provide anoptical amplifier including a function of compensating such thedispersion with a low NF, and being economical with a low electric powerconsumption.

[0014] A second object of the present invention is to provide an opticaltransmission equipment including a function of compensating such thedispersion with a low NF, and being economical with a low electric powerconsumption therewith.

[0015] A third object of the present invention is also to provide anoptical transmission system including a function of compensating suchthe dispersion with a low NF, and being economical with a low electricpower consumption therewith.

[0016] A fourth object of the present invention is to provide a methodfor amplifying an optical signal, including a function of compensatingsuch the dispersion with a low NF, and being economical with a lowelectric power consumption therewith.

[0017] According to the present invention, for dissolving the problemsand for achieving the objects mentioned in the above, there is providedan optical amplifier, for use in an optical transmission equipment,comprising:

[0018] a first optical doped fiber;

[0019] a second optical doped fiber;

[0020] a third optical doped fiber;

[0021] an optical part of bringing loss in the optical signal, and beingprovided between said first optical doped fiber and said second opticaldoped fiber;

[0022] a dispersion compensator being provided between said secondoptical doped fiber and said third optical doped fiber; and

[0023] a pumping light source for pumping being optically connected toso that at least two optical doped fibers are excited in common amongsaid first, second and third optical doped fibers.

[0024] With such the construction mentioned above, the feeble opticalsignal which is weakened on the transmission path(s) due to thepropagation thereof is amplified once before being damaged with the lossof the optical part, such as the optical isolator, etc., withoutdeterioration in the NF thereof, while the first, second and thirdoptical doped fibers for amplifying thereof are pumped or excited incommon, thereby achieving an optical amplifier being economical andsmall-sized with a low electric power consumption.

[0025] Further, according to the present invention, for achieving theobjects mentioned in the above, there is provided an opticaltransmission equipment for transmitting optical signal from an equipmentin upper stream to an equipment of down stream, comprising:

[0026] an optical amplifier portion for amplifying the transmittedoptical signal for transmission thereof; and

[0027] a supervisor/controller portion for receiving information fromthe equipment in upper stream and for sending information including thatof the optical transmission equipment itself to the equipment of downstream, wherein said optical amplifier portion comprises:

[0028] a first optical doped fiber;

[0029] a second optical doped fiber;

[0030] a third optical doped fiber;

[0031] an optical part of bringing loss in the optical signal, and beingprovided between said first optical doped fiber and said second opticaldoped fiber;

[0032] a dispersion compensator being provided between said secondoptical doped fiber and said third optical doped fiber; and

[0033] an pumping light source for pumping at least two optical dopedfibers in common among said first, second and third optical dopedfibers.

[0034] With such the construction mentioned above, the feeble opticalsignal which is weakened on the transmission path(s) due to thepropagation thereof is amplified once before being damaged with the lossof the optical part, such as the optical isolator, etc., withoutdeterioration in the NF thereof, while the first, second and thirdoptical doped fibers for amplifying thereof are pumped or excited incommon, thereby achieving an optical transmission equipment beingeconomical and small-sized with a low electric power consumption.

[0035] Further, according to the present invention, also for achievingthe above-mentioned object, there is provided an optical transmissionsystem for transmitting an optical signal, comprising:

[0036] an optical sender for sending an optical signal converted from anelectric signal onto a transmission path;

[0037] an optical transmission equipment for receiving the opticalsignal being attenuated on said transmission path, and for compensatingwith dispersion by amplification thereof; and

[0038] an optical receiver for the optical signal from said opticaltransmission equipment so as to convert it into the electric signal,wherein, said optical transmission equipment comprises an opticalamplifier portion comprising:

[0039] a first optical doped fiber;

[0040] a second optical doped fiber;

[0041] a third optical doped fiber;

[0042] an optical part of bringing loss in the optical signal, and beingprovided between said first optical doped fiber and said second opticaldoped fiber;

[0043] a dispersion compensator being provided between said secondoptical doped fiber and said third optical doped fiber; and

[0044] an pumping light source for pumping at least two optical dopedfibers in common among said first, second and third optical dopedfibers.

[0045] With such the construction mentioned above, also, the feebleoptical signal which is weakened on the transmission path(s) due to thepropagation thereof is amplified once before being damaged with the lossof the optical part, such as the optical isolator, etc., withoutdeterioration in the NF thereof, while the first, second and thirdoptical doped fibers for amplifying thereof are pumped or excited incommon, thereby achieving an optical transmission system beingeconomical and small-sized with a low electric power consumption.

[0046] Furthermore, according to the present invention, for achievingthe above-mentioned object, there is also provided an opticaltransmission system, comprising:

[0047] a plurality of optical senders, each for sending an opticalsignal of one wavelength converted from a plurality of electric signalonto a transmission path;

[0048] a first transponder for inputting said optical signal of onewavelength to convert into a plurality of optical signals beingdifferent to one another in the wavelength thereof;

[0049] a wavelength multiplexer for multiplexing said plurality ofoptical signals different to one another in the wavelength;

[0050] an optical amplifier for amplifying said multiplexed opticalsignals;

[0051] a wavelength divider for dividing said multiplexed opticalsignals amplified into the plurality of optical signals different to oneanother in the wavelength;

[0052] a second transponder for receiving said the plurality of opticalsignals different to one another in the wavelength to convert into anoptical signal of one wavelength;

[0053] a plurality of an optical receivers, each for converting saidoptical signal of one wavelength into an electric signal, and furtherproviding:

[0054] a supervisor/controller portion at sender side; and

[0055] a supervisor/controller portion at receiver side, wherein saidoptical amplifier has noise figure (NF) being equal or less than 4.5 dB.

[0056] With such the construction mentioned above, also. The feebleoptical signal which is weakened on the transmission path(s) due to thepropagation thereof is amplified once before being damaged with the lossof the optical part, such as the optical isolator, etc., withoutdeterioration in the NF thereof, while the first, second and thirdoptical doped fibers for amplifying thereof are pumped or excited incommon, thereby achieving an optical transmission system beingeconomical and small-sized with a low electric power consumption.

[0057] And, also according to the present invention, there is alsoprovided a method for amplifying an optical signal between twotransmission paths, comprising following steps:

[0058] receiving the optical signal from one of said two transmissionpaths;

[0059] optically amplifying the optical signal with a firstamplification factor, by means of a first optical doped fiber;

[0060] optically suppressing reflection of light due to ASE;

[0061] optically amplifying the optical signal with a secondamplification factor being higher than the first amplification factor,by means of a second optical doped fiber; and

[0062] optically amplifying the optical signal with a thirdamplification factor, by means of a third optical doped fiber, so as tobe transmitted to the other one of said two transmission paths, whereinat least two are pumped in common among said first, second and thirdoptical doped fibers.

[0063] With such the method mentioned above, also, the feeble opticalsignal which is weakened on the transmission path(s) due to thepropagation thereof is amplified once before being damaged with the lossof the optical part, such as the optical isolator, etc., withoutdeterioration in the NF thereof, while the first, second and thirdoptical doped fibers for amplifying thereof are pumped or excited incommon, thereby achieving an optical transmission, economically, with asmall-sized construction and a low electric power consumption.

BRIEF DESCRIPTION OF DRAWINGS

[0064]FIG. 1 is a function block diagram of showing an opticaltransmission equipment for use as a repeater, according to a first andsecond embodiments of the present invention;

[0065]FIG. 2 shows a graph of a measurement result of NF in the opticaltransmission equipment for use as the repeater, according to anembodiment of the present invention;

[0066]FIG. 3 shows a graph for explaining loss compensation withinsertion of an dispersion compensator, in the optical transmissionequipment for use as the repeater, according to an embodiment of thepresent invention;

[0067]FIG. 4 is a function block diagram of showing an opticaltransmission system, as a third embodiment according to the presentinvention;

[0068]FIG. 5 is a function, block diagram of showing an anotherembodiment of the optical amplifier for use as the repeater in theoptical transmission equipment, according to the present invention;

[0069]FIG. 6 is a function block diagram of showing other embodiment ofthe optical amplifier for use as the repeater in the opticaltransmission equipment, according to the present invention; and

[0070]FIG. 7 a function block diagram of showing further otherembodiment of the optical amplifier for use as the repeater in theoptical transmission equipment, according to the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0071] Hereinafter, embodiments according to the present invention willbe fully explained by referring to the attached drawings.

[0072] An optical amplifier, as a first embodiment according to thepresent invention, and an optical transmission equipment, as a secondembodiment thereof, will be explained by referring to FIGS. 1 through 3.

[0073] Referring to FIG. 1 showing the block diagram of the opticaltransmission equipment 100 for use as a relay or repeater, a mainoptical signal and an optical supervising signal, which are multiplexedin the wavelength thereof and are transmitted on a transmission path 20at an upper stream side (at the left-hand side in the figure) bywave-multiplexing, are inputted into a wavelength divider 4, in whichthe supervising signal is divided from main signal. The dividedsupervising signal is converted or exchanged into an electric signal atan O/E (optic/electric) converter portion 40. While, the main signalpasses through the wavelength divider 4, and reaches to a 95:5 coupler 5having a function of coupling the optical signal at a ratio 95:5.Namely, through the 95:5 coupler 5, a large portion (i.e., at the ratio95) of the main signal is transferred to an optical amplifier 10A(indicated by a rectangular of dotted line in the figure) to beamplified therewith, on the other hand, a small portion thereof (i.e.,at the ratio 5) is to a light detector 32 through a narrow band opticalfilter 31. Therefore, the small portion of the main signal is selectedwith a specific wavelength through the narrow band optical filter 31,and is converted or exchanged by the light detector 32 into an electricsignal as indicating the incident light strength thereof. In thisembodiment, the narrow band optical filter 31 is provided since it isassumed that the main signal is one which is multiplexed in thewavelength, however, it is needless to say that the narrow band opticalfilter 31 is not necessary if the main signal is of a single wavelength.

[0074] Also with the main signal which has been amplified in the opticalamplifier 10A (i.e., at the right-hand side in the figure), a smallportion (i.e., at the ratio 5) of it is transferred through an another95:5 coupler 5′ and an another narrow band optical filter 31′ into alight detector 32′ so as to be converted into the electric signal asindicating the incident light strength thereof. While, in a 95:5 coupler4′, on the contrary to the above, the main signal which has beenamplified is multiplexed with the supervising signal again, which isconverted into an optical signal by an E/O (electric/optic) conversionportion 41, so as to be further transmitted to a transmission path 21 ata down stream side (at the right-hand side in the figure).

[0075] The optical amplifier 10A includes a doped fiber portion 1 ofthree (3) stages of doped fibers 1A, 1B and 1C, each being dopes withrear earth, an pumping light source 2 with a light detector 32″ thereof,an optical isolator 6, a dispersion compensator 7, and a plurality ofwavelength multiplexers 3A, 3B and 3C, and a plurality of opticalcouplers 8, 8′, as well. In the construction, an pumping light from thepumping light source 2 is divided by the optical coupler 8 at the ratio8:2 (i.e., a 8:2 coupler 8), and then a small portion of the pumpinglight divided at the ratio 2 is introduced through the wavelengthmultiplexer 3A into the doped fibers 1A for pumping thereof. While, thelarge portion of the pumping light divided at the ratio 8 is furtherdivided into two portion at the ratio 1:1 by the optical coupler (i.e.,a 1:1 coupler) 8′, and they are introduced through the wavelengthmultiplexers 3B and 3C into the doped fibers 1B and 1C, respectively,also for pumping thereof.

[0076] The optical signal which is transmitted to the optical amplifier10A is once amplified by the doped fiber 1A under the condition of lowexcitation. Thereafter, it passes through the optical isolator 6 for useof protection from reflection of ASE (Amplified Spontaneous Emission),and is further amplified by the doped fiber 1B to pass through thedispersion compensator 7. It is very important for this configuration toprotect first optical doped fiber from reflection of ASE. So isolationof isolator 6 is preferable to be kept more than 20 dB, and morepreferably, to be kept more than 30. In general, the signal loss in thedispersion compensator is large, therefore, there is further providedthe doped fiber 1C to amplify the signal again.

[0077] Here, the output of the pumping light source 2 is controlled upona control signal from a supervisor/controller portion 50, which monitorsboth the incident or input light strength and the main signal strengthwhich has been amplified. Further, the strength of the pumping light isconverted into an electric signal in the light detector 32″ of thepumping light source 2 and is provided to the supervisor/controllerportion 50, so as to be monitored thereby. Though the explanation isomitted here from the drawing, however, the temperature of the opticalpumping light source 2, etc. is also monitored.

[0078] The supervisor/controller portion 50 receives the supervisingelectric signal which is converted into the electric signal in theabove-mentioned O/E converter portion 40 so as to extract theinformation being attended to the optical transmission equipment itself.This information includes, such as an information of number ofwavelength multiplexing, an information instructing change of theamplification factor. Namely, to the supervisor/controller portion 50are condensed information from equipment in the upper stream, andmonitor information of the optical transmission equipment itself, so asto be used for controlling the optical pumping light source 2. Further,the supervisor/controller portion 50 multiplexes the monitor informationof the optical transmission equipment itself with the supervisingelectric signal. This supervising electric signal is converted into thesupervising signal in the E/O converter portion 41, and the convertedsignal is multiplexed in the wavelength with the main signal within thewavelength multiplexer 4′ to be sent to the transmission equipment inthe down stream.

[0079] According to the construction mentioned in the above, since thereis provided no such the optical parts, for example the optical isolatoror the like, that brings a large signal loss, at the initial stage ofthe optical amplifier 10A, it is possible to achieve the opticalamplifier and the optical transmission (sender) equipment having a lowNF therein.

[0080] Namely, in the structure of the conventional optical amplifier,the optical isolator is necessary to be provided at the stage in frontof the doped fiber, for protecting from returning back of the reflectionlight caused by the ASE, because of high excitation thereof. However,according to the structure of the present embodiment, even with thedoped fiber 1A is under the condition of low excitation, a sufficientgain can be obtained since the incident signal is low in the levelthereof. Therefore, if the optical isolator is provided between thehighly excited doped fiber 1B and the lowly excited doped fiber 1A, theincident or input signal receives no ill effect on it, in particular, inthe NF thereof, since the incident or input signal to the opticalisolator is already amplified once.

[0081] Also, according to the structure mentioned in the above, thesignal loss following with provision of the dispersion compensator 7 canbe compensated or supplemented by means of the doped fiber 1C.

[0082] Further, with the optical transmission equipment according to thepresent embodiment, since it includes the supervisor/controller portion50, it is also possible to receive the information attending to itself,and to send out the information of itself to the equipment in the downstream as well.

[0083] The input signal to the doped fiber 1A from the transmission path20, in the present construction, lies from −30 dBm up to −5 dBm in thesignal level thereof, and the doped fiber 1A has a gain of 10 dB in thesignal amplification thereof. However, since the isolator is notprovided in the stage in front of the optical isolator 1A as mentionedin the above, an attention must be paid to a phenomenon of anoscillation of light. Accordingly, the gain in the signal amplificationby the doped fiber 1A is preferable to kept to be equal or less than 30dB, and more preferably, within a range from 10 dB to 16 dB. In thisinstance, the doped fiber 1A is sufficient from 3 m to 6 m in the lengththereof. Within such the region of the length mentioned above, the NF inthe doped fiber itself the can be neglected therefrom.

[0084] Further, the gains in the doped fibers 1B and 1C are from 10 dBto 20 dB, respectively, while the lengths are from 10 m to 20 m,respectively.

[0085] Here, for the pumping light source can be used a semiconductorlaser of wavelength 980 nm, or alternatively, that of wavelength 1,480nm in place thereof. However, it is more preferable to use thesemiconductor laser of wavelength 980 nm for it. This is because thenoises generated by the excitation with the pumping light of wavelengthof 940 nm is lower than those with the pumping light of wavelength 1,480nm. Also, the output level of the pumping light is preferable to be from120 mW to 150 mW, for example.

[0086] Moreover, for protecting the doped fiber 1C from the oscillationtherein, it is also possible to provide a further optical isolator inseries to the dispersion compensator 7. As the dispersion compensator 7can be applied a dispersion compensating fiber, a Bragg grating, etc.,for examples.

[0087] A result of measurement on an input/output characteristic and theFN of the optical signal (i.e., input vs. gain & NF test) is shown inFIG. 2, in experiments or tests upon the structure of the opticalamplifier according to the present embodiment mentioned above. Inparticular, FIG. 2 shows the measured values obtained on a system beingconstructed with the elements from the doped fiber 1A to the doped fiber1B.

[0088] Giving explanation on the points at which the measurement aremade in the experiments by referring to FIG. 1, the input (input power)indicates the signal level taken or inputted from the transmission path20, while the gain on the left-hand vertical scale and the NF on theright-hand vertical scale indicate the measured values from the dopedfibers 1A and 1B, respectively, on the system, from the doped fiber 1Ato the doped fiber 1B.

[0089] From FIG. 2, it is apparent that the NF to the input signal fromthe transmission path is regulated to be equal to or less than 3.9 dB.Even by taking into the consideration the fluctuation in the temperatureon the actual system and also unevenness in the sizes on the productionthereof, it is apparent that it can be controlled to be equal to or lessthan 4.5 dB according to the present invention. Further, by compensatingthe fluctuation in the temperature and suppressing the unevenness in thesizes of production, it is also possible to keep it to be equal to orless than 4.0 dB.

[0090] Comparing to the value in the NF according to the conventionaloptical transmission equipment, i.e., 7 dB, the improvement at the value4.0 dB in the NF means, if being converted from the S/N into the signal,that the transmittable distance of the signal can be further extended byabout 100 km, thereby obtaining great improvement therewith.

[0091] Next, by referring to FIG. 3, an explanation will be given on thevalues obtained on the system constructed from the doped fiber 1B to thedoped fiber 1C, in the tests mentioned above. FIG. 3 shows the result ofthe gain and the NF (i.e., noise index) measured on the system mentionedabove, however inserting the optical attenuator in place of thedispersion compensator, for the purpose of obtaining the effect ofbringing the loss in the dispersion compensator variable. The wavelengthmeasured in this test is 1,552 nm, the electric power of the pumpinglight source 50 mW, and the pumping wavelength 980 nm, respectively.

[0092] As is apparent from FIG. 3, in case where the input is equal orless than −20 dB in the signal level thereof, the gain is reduced by 2dB in the system when the attenuation is set at 5 dB with theattenuator, comparing to that obtained when no attenuation is set. And,also the reduction in the gain by 4 dB is obtained in the system, whenthe attenuation is set at 10 dB. On a while, though the NF is largerthan the measured result shown in FIG. 2 since no positive measure istaken for reducing it in this test, however it is appear that it isalmost constant around 5 dB.

[0093] Judging from those test results in the above, it is clearlyindicated that those embodiments according to the present inventionmentioned above have an effect of compensating the losses due to theoptical parts, such as the dispersion compensator, etc. Further, it isalso clearly indicated that those embodiments according to the presentinvention do not bring about the increase in the NF.

[0094] Next, an explanation will be given on a third embodimentaccording to the present invention.

[0095] Referring to FIG. 4 showing the block diagram of an opticaltransmission system of a wavelength multiplex type, according to theembodiment of the present invention; optical signals of a singlewavelength λ0 from a plurality of optical transmitters 201 are convertedinto the plural optical signals of different wavelengths from λ1 to λnin a transponder 210A, and are multiplexed in a wavelengthmultiplexer/de-multiplexer 220. The multiplexed optical signal is, then,amplified with an optical amplifier 10′ at a sender side to be sent outonto the transmission path 20. The optical signal which has been damagedwithin the transmission path is amplified with plural stages of relay orrepeat optical amplifiers 10 to be further sent out onto thetransmission path 21. Namely, the optical signal is amplified by theoptical amplifier 10″ at a receiver side after being amplified with theplural stages of the relay or repeat amplifiers, at the number which isregulated under the NF of each optical amplifier, and then is divided orde-multiplexed in the wavelength multiplexer/de-multiplexer 220 with thewavelength thereof. Further, those signals divided or de-multiplexedinto the signals from λ1 to λn in the wavelength are further convertedback into the optical signals of the single wavelength λ0 in thetransponder 210B to be received by a plurality of optical receivers 202.

[0096] In this instance, an information relating to the conversion inthe wavelengths (i.e., wavelength conversion information) in thetransponder 210A and the wavelength multiplexer/de-multiplexer 220 atthe sender side, is controlled by a supervisor/controller portion 51,and is multiplexed with the main signal, on an output of the opticalsender amplifier 10′ at the receiver side, as the supervising signal. Anoptical repeater amplifier 10, as explained in FIG. 1, divides thesupervising signal from it at the entrance thereof, and the signal ismultiplexed with the main signal at the exit thereof, after beingreproduced and amplified. While, the supervising signal divided at theinput of the optical receiver amplifier 10″ is terminated with thewavelength conversion information thereof in the supervisor/controllerportion 52, then correspondences are made between the respective opticalsignals and the optical receivers onto which they are to be transmitted,by controlling the operations of the wavelengthmultiplexer/de-multiplexer 220B and the transponder 210 at thetransmitter side.

[0097] The transmission distance L between the optical amplifiers isdetermined mainly depending upon the loss in the optical signal on thetransmission path. However, a maximum regenerative relay or repeatdistance L0, i.e., the maximum distance at which the optical signal canbe transmitted only by the optical amplification by means of the opticalamplifier differs greatly depending upon the NF of the opticalamplifier. Namely, the number of the stages of relays or repeaters withthe optical amplifiers having the low NF comes to be larger than thatwith the optical amplifiers having the high NF, therefore, the maximumdistance L0 can be extended by a value making the distance L between theoptical transmitters as a unit thereof.

[0098] Next, an explanation will be given on the optical transmissionapparatus 100 including an another optical amplifier 10B therein,according to the present invention, by referring to FIG. 5.

[0099] In FIG. 5 showing the block diagram of the transmission equipment100 for use as the relay or repeater according to this anotherembodiment, since all of the elements except for the optical amplifier10B are same in the structure thereof, the explanation on the structureand function of the transmission equipment is omitted here.

[0100] As is apparent from the present embodiment, the optical amplifier10B also includes the doped fiber portion 1 of three (3) stages of dopedfibers 1A, 1B and 1C, each being dopes with rear earth, the pumpinglight source 2 with the light detector 32″ thereof, the optical isolator6, and the dispersion compensator 7 therein. In the construction, thepumping light from the pumping light source 2 is also divided by thecoupler 8 at the ratio 8:2 (i.e., a 8:2 coupler), and the small portionof the pumping light at the ratio 2 is introduced through the wavelengthmultiplexer 3A into the doped fibers 1A for pumping thereof. However,the large portion of the pumping light at the ratio 8, in this anotherembodiment, is introduced through the wavelength multiplexers 3B intothe doped fiber 1B for excitation thereof. And, in the presentvariation, the length of the doped fiber 1B and the pumping light at theratio 8 are so designed or selected that the pumping light is in excesstherein. Therefore, the pumping light in excess, passing through thedoped fiber 1B and then bypassing the dispersion compensator 7 at a pairof wavelength multiplexer/de-multiplexers 9A and 9B connectedthereacross, is introduced into the doped fiber 1C for excitationthereof.

[0101] In this construction, the optical signal to be transmitted to theoptical amplifier 10B is amplified with the doped fiber 1A under thecondition of low excitation, once. After that, it passes through theoptical isolator 6 for the protection from the reflection of ASE. Theoptical signal is, further, amplified with the doped fiber 1B and passesthe dispersion compensator 7. Also in this variation, since the signalloss is generally large in the dispersion compensator 7, the opticalsignal is further amplified with the doped fiber 1C.

[0102] And, according to this embodiment, the optical amplifier in theoptical transmission equipment can also obtain the same effects as beobtained with the embodiment in FIG. 1 mentioned above, by paying theattention which was already given previously.

[0103] Next, an explanation will be given on the optical transmissionapparatus 100 including an other optical amplifier 10C therein,according to the present invention, by referring to FIG. 6.

[0104] In FIG. 6 showing the block diagram of the transmission equipment100 for use as the relay or repeater according to this other embodiment,since all of the elements except for the optical amplifier 10C are samein the structure thereof, therefore the explanation on the structure andfunction of the transmission equipment is omitted here.

[0105] In the present other embodiment, the optical amplifier 10C alsoincludes the doped fiber portion 1 of three (3) stages of doped fibers1A, 1B and 1C, each being dopes with rear earth, the pumping lightsource 2 with the light detector 32″ thereof, the optical isolator 6,and the dispersion compensator 7 therein. In the construction, thepumping light from the pumping light source 2 is also divided by theoptical coupler 8 at the ratio 8:2 (i.e., a 8:2 coupler 8), and thesmall portion of the pumping light at the ratio 2 is introduced throughthe wavelength multiplexer 3A into the doped fibers 1A for pumpingthereof. While, the large portion of the pumping light at the ratio 8,in this variation, is introduced through the wavelength multiplexers 3Binto the doped fiber 1B for excitation thereof. And, in the presentother embodiment, the length of the doped fiber 1B and the pumping lightat the ratio 8 are also so combined and designed that the pumping lightis in excess therein. Therefore, the pumping light in excess, afterpassing through the doped fiber 1B, further passes through thewavelength multiplexer/de-multiplexer 9′ to be introduced into the dopedfiber 1C for excitation thereof.

[0106] The optical signal to be transmitted to the optical amplifier 10Cis amplified with the doped fiber 1A under the condition of lowexcitation, once. After that, it passes through the optical isolator 6for the protection from reflection of ASE. The optical signal is,further, amplified with the doped fiber 1B and is divided with thewavelength thereof at the wavelength multiplexer/de-multiplexer 9′ once,and it passes through the dispersion compensator 7. And in this otherembodiment, since the signal loss is generally large in the dispersioncompensator 7, the optical signal brings about the signal loss therein.The optical signal damaged with the loss is turned back to thewavelength multiplexer/de-multiplexer 9′, and is multiplexed inwavelength with the pumping light so as to amplify the signal in thedoped fiber 1C.

[0107] According to the other embodiment, the optical amplifier in theoptical transmission equipment can also obtain the same effects as beobtained with the embodiment in FIG. 1 mentioned above, by paying theattention which was already given previously.

[0108] Next, an explanation will be given on a further other embodimentof the transmission equipment 100 including the optical amplifier 10Ctherein, according to the present invention, by referring to FIG. 7.

[0109] In FIG. 7 showing the block diagram of the transmission equipment100 for use as the relay or repeater, since all of the elements exceptfor the optical amplifier 10D are same in the structure thereof,therefore the explanation on the structure and function of thetransmission equipment is also omitted here.

[0110] In the present further other embodiment, the optical amplifier10D also includes the doped fiber portion 1 of three (3) stages of dopedfibers 1A, 1B and 1C, each being dopes with rear earth, the pumpinglight source 2 and the other pumping light source 2′ with the lightdetector 32″ thereof, the optical isolator 6, and the dispersioncompensator 7 therein. In the construction, the pumping light from thepumping light source 2 excites the doped fiber 1A through the wavelengthmultiplexer 3A. While, the pumping light from the other pumping lightsource 2′ excites the doped fiber 1B through the wavelength multiplexer3B. Also, in the present further other embodiment, the length of thedoped fiber 1B and the pumping light from the other pumping light source2′ are also so combined and designed that the pumping light is in excesstherein. Therefore, the pumping light in excess can pass through thewavelength multiplexer/dd-multiplexer 9′, so as to excite the dopedfiber 1C too.

[0111] The optical signal to be transmitted to the optical amplifier 10Dis amplified with the doped fiber 1A under the condition of lowexcitation, once. After that, it passes through the passes through theoptical isolator 6 for the protection from the reflection of ASE.Further, it is amplified with the doped fiber 1B, and then is divided inthe wavelength thereof at the wavelength multiplexer/de-multiplexer 9′once, and further passes through the dispersion compensator 7. And inthis further other embodiment, since the signal loss is also generallylarge in the dispersion compensator 7, the optical signal brings aboutthe signal loss therein. The optical signal damaged with the loss isturned back to the wavelength multiplexer/de-multiplexer 9′, and ismultiplexed in wavelength with the pumping light so as to amplify thesignal in the doped fiber ID.

[0112] Here, the output of the other pumping light source 2′ iscontrolled by a control signal from the supervisor/controller portion 50which monitors the strengths in the input light and the amplified mainsignal. Further, the strength in the pumping light is converted into theelectric signal by the light detector 32″ of the other pumping lightsource 2′, thereby being monitored also by the supervisor/controllerportion 50.

[0113] In this further other embodiment, differing from the variousembodiments mentioned in the above, there are contained two set of theexcitation light sources. However, since the pumping light source from30 mW to 50 mW in the power thereof is enough for use for pumping thedoped fiber 1A of the low excitation type, therefore it is cheap in theprice.

[0114] Also, according to this further other embodiment, the opticalamplifier in the optical transmission equipment can also obtain the sameeffects as be obtained with the embodiment in FIG. 1 mentioned above, bypaying the attention which was already given previously.

[0115] In the further other embodiment, as was mentioned, the dopedfiber of three stages is excited by the two set of the pumping lightsources. However, the present invention should not be limited only tothe embodiments mentioned in the above. For example, it is furtherpossible to excite or pump the doped fibers 1A and 1B by means of oneset of the pumping light source, as well as to make the doped fiber 1Cas a single pumping light source. In this instance, it is preferable touse the pumping light of the wavelengths 980 nm for pumping the dopedfibers 1A and 1B. Though not only the pumping light of the wavelengths980 nm but also of 1,480 nm can be used, and it is possible to obtain ahigh output by selecting the pumping light to be 1,480 nm in thewavelength thereof.

[0116] In all of those embodiments mentioned in the above, therelationship between the doped fibers and the pumping light source(s) isnot restricted with the construction shown in the figures. Namely, itmay be constructed with a front excitation method,in which the signaland the excitation light are in the same direction, or with a rearexcitation method in which the signal and the excitation light arereversed in the directions thereof, or with a method of exciting in bothdirections.

[0117] Further, according to the present invention, it is also possibleto provide the optical amplifier and the optical transmission equipmentby using thereof, with which the high speed optical signal can beamplified with a cheap price and a low NF can be obtained. Furthermore,it is also possible to provide the optical transmission system enablingthe long distance transmission.

What is claimed is:
 1. An optical amplifier, for use in an opticaltransmission equipment, comprising: a first optical doped fiber; asecond optical doped fiber; a third optical doped fiber; an optical partof bringing loss in the optical signal, and being provided between saidfirst optical doped fiber and said second optical doped fiber; adispersion compensator being provided between said second optical dopedfiber and said third optical doped fiber; and a pumping light source forpumping being optically connected to so that at least two optical dopedfibers are excited in common among said first, second and third opticaldoped fibers.
 2. An optical amplifier, as defined in the claim 1,wherein said pumping light source is optically connected to so that saidsecond and third optical doped fibers are excited in common.
 3. Anoptical amplifier, as defined in the claim 1, wherein the loss broughtin the optical signal by said optical part is noise figure.
 4. Anoptical amplifier, as defined in the claim 1, wherein said optical partincludes an optical isolator being provided between said first opticaldoped fiber and said second optical doped fiber.
 5. An opticalamplifier, as defined in the claim 1, wherein length of said firstoptical doped fiber is shorter than that of both of said second andthird optical doped fibers.
 6. An optical amplifier, as defined in theclaim 1, wherein noise figure of said optical amplifier is set to beequal or less than 6 dB.
 7. An optical transmission equipment fortransmitting optical signal from an equipment in upper stream to anequipment of down stream, comprising: an optical amplifier portion foramplifying the transmitted optical signal for transmission thereof; anda supervisor/controller portion for receiving information from theequipment in upper stream and for sending information including that ofthe optical transmission equipment itself to the equipment of downstream, wherein said optical amplifier portion comprises: a firstoptical doped fiber; a second optical doped fiber; a third optical dopedfiber; an optical part of bringing loss in the optical signal, and beingprovided between said first optical doped fiber and said second opticaldoped fiber; a dispersion compensator being provided between said secondoptical doped fiber and said third optical doped fiber; and an pumpinglight source for pumping at least two optical doped fibers in commonamong said first, second and third optical doped fibers.
 8. An opticaltransmission equipment, as defined in the claim 7, wherein said pumpinglight source of said optical amplifier portion is optically connected toso that said second and third optical doped fibers are excited incommon.
 9. An optical amplifier, as defined in the claim 7, wherein theloss brought in the optical signal by said optical part is noise figure.10. An optical transmission equipment, as defined in the claim 7,wherein said optical part of said optical amplifier portion includes anoptical isolator being provided between said first optical doped fiberand said second optical doped fiber.
 11. An optical transmissionequipment, as defined in the claim 7, wherein length of said firstoptical doped fiber is shorter than that of both of said second andthird optical doped fibers.
 12. An optical transmission equipment, asdefined in the claim 7, wherein noise figure of said optical amplifierportion is set to be equal or less than 6 dB.
 13. An opticaltransmission system for transmitting an optical signal, comprising: anoptical sender for sending an optical signal converted from an electricsignal onto a transmission path; an optical transmission equipment forreceiving the optical signal being attenuated on said transmission path,and for compensating with dispersion by amplification thereof; and anoptical receiver for the optical signal from said optical transmissionequipment so as to convert it into the electric signal, wherein, saidoptical transmission equipment comprises an optical amplifier portioncomprising: a first optical doped fiber; a second optical doped fiber; athird optical doped fiber; an optical part of bringing loss in theoptical signal, and being provided between said first optical dopedfiber and said second optical doped fiber; a dispersion compensatorbeing provided between said second optical doped fiber and said thirdoptical doped fiber; and an pumping light source for pumping at leasttwo optical doped fibers in common among said first, second and thirdoptical doped fibers.
 14. An optical transmission system, as defined inthe claim 13, wherein said pumping light source of said opticalamplifier portion is optically connected to so that said second andthird optical doped fibers are excited in common.
 15. An opticalamplifier, as defined in the claim 13, wherein the loss brought in theoptical signal by said optical part within said optical amplifierportion is noise figure.
 16. An optical transmission system, as definedin the claim 13, wherein said optical part of said optical amplifierportion includes an optical isolator being provided between said firstoptical doped fiber and said second optical doped fiber.
 17. An opticaltransmission system, as defined in the claim 13, wherein length of saidfirst optical doped fiber is shorter than that of both of said secondand third optical doped fibers.
 18. An optical transmission equipment,as defined in the claim 13, wherein noise figure of said opticalamplifier portion is set to be equal or less than 6 dB.
 19. An opticaltransmission system, comprising: a plurality of optical senders, eachfor sending an optical signal of one wavelength converted from aplurality of electric signal onto a transmission path; a firsttransponder for inputting said optical signal of one wavelength toconvert into a plurality of optical signals being different to oneanother in the wavelength thereof; a wavelength multiplexer formultiplexing said plurality of optical signals different to one anotherin the wavelength; an optical amplifier for amplifying said multiplexedoptical signals; a wavelength divider for dividing said multiplexedoptical signals amplified into the plurality of optical signalsdifferent to one another in the wavelength; a second transponder forreceiving said the plurality of optical signals different to one anotherin the wavelength to convert into an optical signal of one wavelength; aplurality of an optical receivers, each for converting said opticalsignal of one wavelength into an electric signal, and further providing:a supervise/controller portion at sender side; and asupervise/controller portion at receiver side, wherein said opticalamplifier has noise figure (NF) being equal or less than 4.5 dB.
 20. Amethod for amplifying an optical signal between two transmission paths,comprising following steps: receiving the optical signal from one ofsaid two transmission paths; optically amplifying the optical signalwith a first amplification factor, by means of a first optical dopedfiber; optically suppressing reflection of light due to ASE; opticallyamplifying the optical signal with a second amplification factor beinghigher than the first amplification factor, by means of a second opticaldoped fiber; and optically amplifying the optical signal with a thirdamplification factor, by means of a third optical doped fiber, so as tobe transmitted to the other one of said two transmission paths, whereinat least two are pumped in common among said first, second and thirdoptical doped fibers.